There are three states being gas, liquid and solid. The state of an element will depend on the temperature and pressure. Here is an example of the state of metal:
- As a metallic gas, particles exist as single atoms otherwise known as 'monatomic'.
- As the temperature falls, the metal condenses to a liquid metal (at the boiling point) which has weak forces between the atoms which allows the atoms to flow around each other.
- When the metal cools even further, the metal turns to a solid where the atoms align themselves in a lattice structure (or a crystalline structure).
- An example of an amorphous solid is Quartz which has a lattice structure.
- However, when melted, with silicon dioxide to make glass and supercooled, the crystalline structure cannot be formed. Instead, the atoms arrange themselves randomly and an amorphous solid is formed.
- When reheated, the solid does not change phase from a solid to liquid but gradually softens instead (phase doesn't change sharply).
- For amorphous materials, the properties of it are the same in every direction which is also known as having an isotropic property.
- Cannot control dislocations/cracks direction in amorphous solids. Take glass - the crack expands randomly.
- The energy required is reduced when atoms are packed closer together to do something on an atomic or microscopic level (i.e. shift some atoms).
- The amount of stabilization achieved by anchoring reactions between particles is greater. A crystalline lattice is formed.
- FCC are more ductile.
- BCC are typically stronger.
Hexagonal Packed Structures
A useful tip to remember is that it is difficult to cast iron with the same size crystals because different parts of iron will cool at different rates and, therefore, produce different size crystals.
- Contraction of a liquid as it cools prior to its solidification.
- Contraction during phase change from a liquid to solid.
- Contraction of the solid as it continues to cool to ambient temperature.
- Point defects which an atom is missing or placed irregularly in a lattice structure.
- Linear defects which are groups of atoms that are in irregular positions. This is commonly more known as dislocations.
- Planar defects which are interfaces between homogeneous regions of the material. Planar defects include grain boundaries, stacking faults and external surfaces.
- Self interstitial atom which is an atom that has crowded its way into a interstitial void in the crystalline structure.
- Substitutional impurity atom which is an atom of a different type than the bulk atoms, which has replaced one of the bulk atoms in the lattice. They are usually close in size (within around 15%) to the bulk atom.
- Interstitial impurity atoms are much smaller than the atoms in the bulk matrix and fit into the open space of the bulk atoms of the lattice structure. An example of this is the carbon atoms that are added to iron to make steel.
Linear Defects and Dislocations
- Ones that have not yet moved from their original position.
- Ones that have moved to a new position and have re-established metallic bonds.
- Ones that are in the process of moving.